1. Field of the Invention
The present invention relates to hydraulic systems that operate actuators, such as cylinder/piston arrangements, and more particularly to hydraulic systems that operate actuators in powered and regenerative modes.
2. Description of the Related Art
A wide variety of machines are operated by a hydraulic system with a plurality of hydraulic actuators, such as cylinder connected one component of the machine and a piston coupled by a rod to another component. The piston divides the interior of the cylinder into two internal chambers and alternate application of hydraulic fluid under pressure to each chamber moves the piston in opposite directions, thereby moving the two components with respect to each other.
In a common hydraulic system, flow of hydraulic fluid to the cylinder was controlled by a manually operated valve in which the human operator moved a lever that was mechanically connected to a spool within a bore of the valve, as shown in U.S. Pat. No. 5,579,643. Movement of that lever placed the spool into various positions with respect to cavities in the bore that communicated with a supply conduit from a pump, return conduit to a fluid tank, and conduits to the chambers of the associated cylinder. Moving the spool in one direction controlled flow of pressurized hydraulic fluid from the pump to one cylinder chamber and allowed fluid in the other chamber to flow to the tank. This drove the piston and the rod connected thereto in one direction. Moving the spool in the opposite direction reversed the fluid flow with respect to the cylinder chambers producing motion in the opposite direction. Varying the amount that the spool was moved in the appropriate direction changed the size of a metering orifice and thus the rate at which fluid flows to the associated cylinder chamber, thereby driving the piston at proportionally different speeds. A pressure compensation mechanism often was incorporated into the spool valve assembly to provide a substantially constant pressure drop across the metering orifice.
There is a trend away from manually operated hydraulic valves toward electrically controlled solenoid valves. U.S. Pat. No. 6,637,461 describes a spool valve that is pilot operated by a pair of electrohydraulic valves to control bidirectional motion of the valve spool.
With both manually and electrically operated devices, the spool valve was built into a separate body, commonly referred to as a valve section, and the valve sections for the plurality of machine functions were bolted side by side to form a valve assembly at the operator workstation of the machine. Each valve section had workports for connecting to the chambers of the respective cylinder. Each valve section also had passages there through for the supply conduit, the tank return conduit, and a load sense circuit, wherein those passages aligned with similar passages in adjacent valve sections to convey fluid through the entire valve assembly. End sections of the valve assembly had ports to connect the supply and tank conduits as well as apertures in which pressure relief valves were mounted.
An alternative to a spool valve comprised a Wheatstone bridge arrangement of four proportional electrohydraulic valves with each one connected between two different corners of a square. Two opposing corners were connected to the workports for the two cylinder chambers. One remaining corner of the bridge was coupled to the supply conduit and the last corner was connected to the tank return conduit. During powered extension and retraction modes of operating the hydraulic cylinder, two valves on opposite sides of the bridge were opened so that fluid from the supply conduit flowed into one cylinder chamber and all the fluid exiting the other cylinder chamber flowed to the tank return conduit.
In an overriding load condition, the external load or other force acting on the machine causes extension or retraction of the hydraulic actuator without requiring significant pressure from the supply conduit. That force drove fluid out of one cylinder chamber, while expansion of the other chamber drew fluid from the supply conduit. During this condition, fluid exited the cylinder under relatively high pressure, thereby containing energy that was lost when the fluid was released into the tank.
The Wheatstone bridge arrangement had the advantage over a spool valve of enabling operation in a regeneration mode in which the energy of that exhausting fluid was recycled, instead of being released unused into the tank. In a self regeneration mode, the two adjacent valves connected to either the supply conduit corner of the bridge of the tank return conduit corner were opened while the other valves remained closed. Thus fluid exhausting from one cylinder chamber is routed by the two of the proportional electrohydraulic valves to the other cylinder chamber that is expanding. As a result, the fluid exiting the contracting cylinder chamber flowed into and was used to fill the expanding chamber, thereby reducing or eliminating the quantity of fluid required from the supply conduit. This required that two proportional electrohydraulic valves had to be accurately controlled to properly meter the regeneration flow. Thus, the electric currents applied to open both valves to precise and consistent positions. In addition, the regeneration flow encountered an energy loss in each of the two valves. One attempt at reducing magnitude of that energy loss involved connecting a fifth electrohydraulic valve directly between the two workports of the valve bridge. Nevertheless, energy losses in the hoses between the valve assembly and the cylinder still affected the efficiency of the regeneration mode.
It is desirable to provide a low energy loss regeneration mode on a hydraulic system that employs spool valves at the operator workstation. This enables an existing machine design to be updated with a regeneration mode of operation.